1. Field of the Invention (Technical Field)
The present invention relates to ultrasonic inspection of structural failures particularly for detecting disbonds or delaminations.
2. Background Art
One of the primary goals of the Federal Aviation Administration's (FAA) National Aging Aircraft Research Program (NAARP) is to foster new technology associated with the repair of civil aircraft. A typical aircraft can experience over 2,000 fatigue cycles (cabin pressurizations) and many more flight hours in a single year. The unavoidable by-product of this use is that flaws develop throughout the aircraft's skin and substructure elements. These flaws can take the form of cracks, corrosion, disbonds, dents, and gouges.
Composite doublers, or repair patches, provide a repair technique that can enhance the way aircraft are maintained. The high modulus of Boron-Epoxy composite material enables a doubler to pick up load efficiently and effectively when bonded to a metal structure. The load transfer occurs by shear through the adhesive. The composite doubler repair has several advantages over conventional repairs which include: corrosion resistance; light weight/high strength; elimination of rivets and additional rivet holes in the skin; conformation to complex shapes; access only to the outside of the fuselage being needed; and substantial cost and time savings.
As the commercial airline industry responds to calls for the ensured airworthiness of global airline fleets, inspection reliability is of growing importance. The development and application of new Nondestructive Inspection (NDI) techniques needs to keep pace with the growing understanding of aircraft structural aging phenomena. A primary inspection requirement for these doublers is the identification of disbonds between the composite laminate and the aluminum parent material.
Acceptance of composite doublers by civil aviation industry depends highly on a quick and comprehensive assessment of the integrity of the doubler at the initial installation of the composite doubler and at regular inspection intervals of the aircraft. In particular, identification of disbonds between the doubler and the aluminum skin and delaminations within the composite are important since these defects prevent the doubler from performing as designed. Disbonds can occur at installation of the doubler or at anytime during the service life of the aircraft. Because of the rapidly increasing use of composites on commercial airplanes, coupled with the potential for economic savings associated with their use in aircraft structures, it appears that the demand for validated composite inspection techniques will increase.
The two main potential causes of structural failure in composite doubler installations are cracks in the aluminum and adhesive disbonds/delaminations. When disbonds or delaminations occur, they may lead to joint failures. By their nature, they occur at an interface and are, therefore, always hidden. A combination of fatigue loads and other environmental weathering effects can combine to initiate these types of flaws. Periodic inspections of the composite doubler for disbonds and delaminations (from fabrication, installation, fatigue, or impact damage) is essential to assuring.the successful operation of the doubler over time. The interactions at the bond interface are extremely complex, with the result that the strength of the bond is difficult to predict or measure. Even a partial disbond may compromise the integrity of the structural assembly. Therefore, it is necessary to detect all areas of disbonding or delamination before joint failures can occur.
Ultrasonic inspection is a nondestructive method in which beams of high frequency sound waves are introduced into materials for the detection of surface and subsurface flaws in the material. In ultrasonic pulse-echo inspections, short bursts of ultrasonic energy are interjected into a testpiece at regular intervals of time. In most pulse-echo systems, a single transducer acts alternately as the sending and receiving transducer. Sometimes it is advantageous to use separate sending and receiving transducers for pulse-echo inspection. The term pitch-catch is often used in connection with separate sending and receiving transducers.
The sound waves, normally at frequencies between 0.1 and 25 MHz, travel through the material with some attendant loss of energy (attenuation) and are reflected at interfaces. The reflected beam is displayed and then analyzed to define the presence and location of flaws. The degree of reflection depends largely on the physical state of the materials forming the interface. Cracks, delaminations, shrinkage cavities, pores, disbonds, and other discontinuities that produce reflective interfaces can be detected. Complete reflection, partial reflection, scattering or other detectable effect on the ultrasonic waves can be used as the basis of flaw detection. In addition to wave reflection, other variations in the wave, which can be monitored, include: time of transit through the structure to be inspected, attenuation, and features of the spectral response.
The principal advantages of ultrasonic inspection as compared to other NDI techniques are: superior penetrating power for detection of deep flaws; high sensitivity permitting the detection of extremely small flaws; accuracy in determining size and position of flaws; only one surface need be accessible; nonhazardous operations with no effect on personnel and equipment nearby; portability; and output that can be digitally processed. However, conventional pulse-echo techniques using flat or normal focus transducers are not effective in characterizing the disbond condition of a composite/aluminum interface. For example, when using 2" diameter focus transducers, the echo amplitude change observed at the disbond may be only slightly different than that due to unflawed portions of the composite bond-line.
The present invention overcomes these difficulties encountered with prior art flat transducer ultrasonics inspection. In the preferred embodiment, a noticeable improvement in the pulse echo response can be obtained by using a transducer with a focus-lens and by placing a stop in the center of the transducer. A discussion of this technology is included in the article entitled "Ultrasonic Inspection Technique for Composite Doubler/Aluminum Skin Bond Integrity for Aircraft," authored by J. Gieske, D. Roach, and P. Walkington, and also in Sandia Report SAND98-1014, UC 906 entitled "Development and Validation of Nondestructive Inspection Techniques for Composite Doubler Repairs on Commercial Aircraft," authored by D. Roach and P. Walkington, and the disclosures therein are herein incorporated by reference.